1. Field of the Invention
The present invention relates to a solid electrolytic capacitor used for a power supply circuit etc.
2. Description of Related Art
There has been a solid electrolytic capacitor using tantalum, niobium, or the like as a valve metal. Such a solid electrolytic capacitor is small, has a large capacitance, excellent frequency characteristics, and low ESL (equivalent series inductance) characteristics and is widely used for a decoupling circuit, a power supply circuit or the like of a CPU (Central Processing Unit).
Commercial production of the solid electrolytic capacitor of especially a substrate electrode structure type is progressing with development of a portable electronic device. When this kind of solid electrolytic capacitor is mounted on an electronic circuit board, the structure of a connection part between a terminal part and a mounting board which is called a solder fillet becomes important along with the terminal part of the mounting electrode surface.
Japanese Unexamined Patent Application Publication No. 2004-103981 discloses a technique of forming recesses on a side surface part of an anode terminal and a cathode terminal in a solid electrolytic capacitor. These recesses are created to the mounting surface side or the mounting surface side and the opposite side as well. When the solid electrolytic capacitor is mounted to a mounting substrate using solder, from the mounting surface side to a bottom surface of the recesses, the solder bonds the bottom surface and the mounting substrate.
Japanese Unexamined Patent Application Publication No. 2008-258602 discloses a technique of forming a concave shape fillet formation surface (codes 15e and 15f in FIG. 8 of Japanese Unexamined Patent Application Publication No. 2008-258602) that vertically penetrates in the side surface of a conversion substrate of the lower surface electrode solid electrolytic capacitor. In detail, a cutout section is provided on a surface exposed to an outside surface. The conversion substrate, which has an anode terminal formation part and a cathode terminal formation part of which inside is plated, is used for connection to a capacitor element. After outer coating resin (the code 19 of FIG. 8 of Japanese Unexamined Patent Application Publication No. 2008-258602) is formed, cutting is performed along cutting surfaces, thus a fillet formation surface is formed on the outside surfaces of the anode and cathode of the lower surface electrode type solid electrolytic capacitor.
Japanese Unexamined Patent Application Publication No. 2008-270317 discloses a technique of providing the fillet formation part in a side surface concave part of an insulating plate. Further, Japanese Unexamined Patent Application Publication No. 2006-190925 discloses a technique of giving rise to low ESL characteristics by reducing a lead-out distance to a mounting surface of an anode leadframe and a cathode leadframe.
As a smaller size is required for the solid electrolytic capacitor, it is necessary to improve volumetric efficiency of a capacitor element for an outline of the solid electrolytic capacitor in order to further reduce the size. However, there are problems in the structure with the recesses in the leadframes (as in Japanese Unexamined Patent Application Publication No. 2004-103981) and the structure with the fillet formation part in the electrode substrate (as in Japanese Unexamined Patent Application Publication Nos. 2008-258602 and 2008-270317), thus the inventors have found a problem that it is difficult to provide stable fillet formation while improving the problems.
In the case of the structure of Japanese Unexamined Patent Application Publication No. 2004-103981, as a partly L-shaped manufacturing frame (leadframe) is embedded in the outer coating resin, the volumetric efficiency is reduced. Further, the outer coating resin will leak to the leadframe mounting terminal surface, and a failure is generated at the time of mounting to the circuit substrate.
Japanese Unexamined Patent Application Publication No. 2004-103981 discloses the structure in which the surface (concave part) exposed to the outside surface of the anode and the cathode of the capacitor is plated and the fillet is formed in the plated part. However, a part including the fillet is limited to the thickness of the leadframe, and the height for forming the fillet may be insufficient.
In the case of the structure of Japanese Unexamined Patent Application Publication No. 2008-258602, the solder gets wet in the fillet formation surface. However, the electrode substrate projects outside the outer coating resin, and the size of the outer coating resin is limited. Therefore, the cathode area is reduced and thereby generating a disadvantageous structure for capacity increase.
Moreover, the second anode upper terminal (code 25) shown in FIG. 1 of Japanese Unexamined Patent Application Publication No. 2008-270317 has the structure that does not extends to the product end surface.
This is because that the gas generated (steam or solvent volatile content from resin) from inside the capacitor terminal due to the thermal history through reflow at the time of product mounting cannot sufficiently escape from inside the capacitor element as the insulating plate and the outer coating resin are adhered. Accordingly, gas pressure may cause warpage of the insulating plate and delamination between the insulating plate and the outer coating resin.
Japanese Unexamined Patent Application Publication No. 2006-190925 discloses a technique of using a leadframe structure aiming for low ESL. In the case of the leadframe structure, processing is difficult and it takes more time to change a mounting terminal and an inner terminal involved with the change in product size as compared with the substrate structure, thus there is a difficulty in mass productivity. Additionally, in the leadframe structure, the outer coating resin may wrap around the electrode terminal of the mounting surface side at the time of forming the external part, and there has been a solder wettability problem of the external terminal when the capacitor is mounted to a substrate.
Therefore, a purpose of the present invention is to provide a solid electrolytic capacitor that solves the abovementioned problem, is excellent in productivity, improves the volumetric efficiency aiming to increase the capacity, and has a stable fillet shape at the time of mounting.